LinkedBlockingDeque是双向链表实现的双向并发阻塞队列。该阻塞队列同时支持FIFO和FILO两种操作方式，即可以从队列的头和尾同时操作(插入/删除)；并且，该阻塞队列是支持线程安全。此外，LinkedBlockingDeque还是可选容量的(防止过度膨胀)，即可以指定队列的容量。如果不指定，默认容量大小等于Integer.MAX_VALUE。
简单来讲就是一个可以有固定长度的,有锁的linkedList.本文采用的是1.7的jdk,以下是对jdk源代码的分析.
/** 这是一个内部类,与linkedList相似,这个就是他的一个链表的核心,在保存元素的同时,指定了上一个下一个元素*/
static final class Node<E> {
/**
*当前元素
*/
E item;
Node<E> prev;
Node<E> next;
Node(E x) {
item = x;
}
}
transient Node<E> first;
transient Node<E> last;
/** 元素数量 */
private transient int count;
/** 队列最大值 */
private final int capacity;
以下是三个关键,全局的lock和用于take和put的两个工具.注意这里的take和put指的是取和存的所有操作
/** Main lock guarding all access */
final ReentrantLock lock = new ReentrantLock();
/** Condition for waiting takes */
private final Condition notEmpty = lock.newCondition();
/** Condition for waiting puts */
private final Condition notFull = lock.newCondition();
然后是一堆构造器
public LinkedBlockingDeque() {
this(Integer.MAX_VALUE);
}
public LinkedBlockingDeque(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
}
//把collection内的元素添加到尾部
public LinkedBlockingDeque(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
final ReentrantLock lock = this.lock;
lock.lock(); // Never contended, but necessary for visibility
try {
for (E e : c) {
if (e == null)
throw new NullPointerException();
if (!linkLast(new Node<E>(e)))
throw new IllegalStateException("Deque full");
}
} finally {
lock.unlock();
}
}
然后是两个追加方法,读着两个方法需要注意几个问题,第一就是这个属于实现方法只是实现添加功能,而把锁和阻塞的处理完全交给了上层.而通过上衣构造器我们可以看到对于阻塞队列满处理的方式就是直接抛错,队列已满.notEmpty.signal();这句话是用来唤醒notEmpty,这里就涉及到高级锁ReentrantLock的api,其本身不能产生wait和notify的效果而只能用工具类condition的await和signal.查看signal的源码可以知道这里只是一个唤醒而不是状态位的记录,所以对于运行中的lock是不会产生任何效果.
/**
* Links node as first element, or returns false if full.
*/
private boolean linkFirst(Node<E> node) {
// assert lock.isHeldByCurrentThread();
if (count >= capacity)
return false;
Node<E> f = first;
node.next = f;
first = node;
if (last == null)
last = node;
else
f.prev = node;
++count;
notEmpty.signal();
return true;
}
/**
* Links node as last element, or returns false if full.
*/
private boolean linkLast(Node<E> node) {
// assert lock.isHeldByCurrentThread();
if (count >= capacity)
return false;
Node<E> l = last;
node.prev = l;
last = node;
if (first == null)
first = node;
else
l.next = node;
++count;
notEmpty.signal();
return true;
}
//接着是三个移除方法,通过指针的重新指向,将首尾元素重新指定
/**
* Removes and returns first element, or null if empty.
*/
private E unlinkFirst() {
// assert lock.isHeldByCurrentThread();
Node<E> f = first;
if (f == null)
return null;
Node<E> n = f.next;
E item = f.item;
f.item = null;
f.next = f; // help GC
first = n;
if (n == null)
last = null;
else
n.prev = null;
--count;
notFull.signal();
return item;
}
/**
* Removes and returns last element, or null if empty.
*/
private E unlinkLast() {
// assert lock.isHeldByCurrentThread();
Node<E> l = last;
if (l == null)
return null;
Node<E> p = l.prev;
E item = l.item;
l.item = null;
l.prev = l; // help GC
last = p;
if (p == null)
first = null;
else
p.next = null;
--count;
notFull.signal();
return item;
}
/**
* Unlinks x.
*/
void unlink(Node<E> x) {
// assert lock.isHeldByCurrentThread();
Node<E> p = x.prev;
Node<E> n = x.next;
if (p == null) {
unlinkFirst();
} else if (n == null) {
unlinkLast();
} else {
p.next = n;
n.prev = p;
x.item = null;
// Don't mess with x's links. They may still be in use by
// an iterator.
--count;
notFull.signal();
}
}
四个对首尾的添加方法
/**
* @throws IllegalStateException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public void addFirst(E e) {
if (!offerFirst(e))
throw new IllegalStateException("Deque full");
}
/**
* @throws IllegalStateException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public void addLast(E e) {
if (!offerLast(e))
throw new IllegalStateException("Deque full");
}
/**
* @throws NullPointerException {@inheritDoc}
*/
public boolean offerFirst(E e) {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
return linkFirst(node);
} finally {
lock.unlock();
}
}
/**
* @throws NullPointerException {@inheritDoc}
*/
public boolean offerLast(E e) {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
return linkLast(node);
} finally {
lock.unlock();
}
}
下面注意这两个方法,notFull.await();通过notFull.await,当添加失败的时候向condition里面加入,当notEmpty.signal();的时候我们看一下他的操作.是获得一个首元素然后LockSupport.unpark(node.thread);解锁这个元素,也就是说每signal一次就取出一个元素unpark
/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public void putFirst(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
while (!linkFirst(node))
notFull.await();
} finally {
lock.unlock();
}
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public void putLast(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
while (!linkLast(node))
notFull.await();
} finally {
lock.unlock();
}
}
以下是两个限时插入,这里请先看一个文献https://www.zhihu.com/question/36771163, lock.lockInterruptibly();是获得锁并且这个锁是可以相应中断的.与lock相比只是多了个标识位而已.如果没有外部的线程interrupt打断的话,就没有区别.当线程Thread调用interrupt()的时候就会强制stop线程.并抛出错.
/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public boolean offerFirst(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (!linkFirst(node)) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
return true;
} finally {
lock.unlock();
}
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws InterruptedException {@inheritDoc}
*/
public boolean offerLast(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
Node<E> node = new Node<E>(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (!linkLast(node)) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
return true;
} finally {
lock.unlock();
}
}
剩余部分的api基本都是跟以上类似就不一一阐述了......